1. Field of the Invention
The present invention relates to a data conversion apparatus and its method and, more particularly, to a data conversion method suitable for digital arithmetic operations, and a data conversion apparatus using the method.
2. Description of the Related Art
The volume of image data digitally processed by computers and the like is increasing each year. Also, higher quality upon forming a color image is increasingly required each year. In order to form a high-quality color image, color conversion is indispensable, and requires high-speed processing in addition to high precision and implementation of flexible conversion characteristics.
Since color information generally forms a three-dimensional space, color conversion determines the correspondence of color information from a given color space to another color space. Many schemes for attaining such conversion are available. Among these schemes, color conversion that combines a look-up table (LUT) and interpolation (Japanese Patent Laid-Open Nos. 53-123201 and 8-237497) is prevalently used. Also, color conversion that combines an LUT and interpolation includes various schemes. In consideration of the required data size, computation volume, continuity of outputs among unit rectangular hexahedra, gray line interpolation characteristics, and the like, tetrahedral interpolation disclosed in Japanese Patent Laid-Open No. 53-123201 is most suitable.
However, the tetrahedral interpolation disclosed in Japanese Patent Laid-Open No. 53-123201 can be directly applied only when the unit rectangular hexahedron is a regular hexahedron. Upon implementing color conversion by a digital computation processing apparatus, the limitation of a unit rectangular hexahedron to a regular hexahedron cannot be ignored in implementation of a conversion processing apparatus.
This limitation will be described in detail below. In a conversion processing apparatus that performs digital processing, in order to convert all unit rectangular hexahedra into regular hexahedra, the grid spacing is limited, and the conversion precision and data size (the number of grid points) cannot be optimized. For example, if input data is 8-bit data (0 to 255), the grid spacing must be set at one of 85 (the number of grid points=4), 51 (6), 17 (16), 15 (18), 5 (32), and 3 (86) to convert all unit rectangular hexahedra into regular hexahedra.
On the other hand, since the color space is not uniform in the entire area, it is effective for improving the conversion precision of a specific area to intentionally set a small grid spacing of that area. For example, upon conversion from RGB to CMYK, a gray level drop between grids arising from an undercolor removal (UCR) process readily occurs in a dark area, i.e., (R, G, B)=(0, 0, 0). However, this problem can be effectively solved by setting a small grid spacing of that area. Japanese Patent Laid-Open Nos. 7-131668, 10-70669, and the like disclose schemes for improving the conversion precision of a specific color area by setting a small grid spacing. However, the schemes described in these references require an area discrimination process for changing the processes depending on the areas of input color information, and are inferior to the technique disclosed in Japanese Patent Laid-Open No. 53-123201 above in terms of the processing speed and circuit scale.